NiCrAlY Coatings Research Articles

The Effect of Spray Distance on the Hot Corrosion Behavior of HVOF‐Sprayed NiCrAlY Coatings in a Simulated Gas Turbine Environment

ABSTRACTTo utilize the unique properties of coatings produced using the high‐velocity oxy‐fuel (HVOF) method, this study examined the effect of spray distance on the hot corrosion behavior of NiCrAlY coatings deposited on a steel substrate. The coating samples were obtained at spray distances of 20, 25, and 30 cm. The flow rates of oxygen and propane were maintained constant at 300 and 60 lit/min, respectively. Additionally, the powder feeding rate was 32 g/min. The samples were then subjected to the hot corrosion test in a salt mixture of Na2SO4−60% V2O5 at 900°C in an ambient atmosphere. The surface and the cross section of the samples were observed using a scanning electron microscope (SEM) and the chemical composition of different sections was determined by energy‐dispersive spectroscopy (EDS). The phases were also characterized using X‐ray diffraction analysis (XRD). The best corrosion resistance was found in the sample coating formed at a spraying distance of 20 cm, as the results showed that increasing the spraying distance led to a reduction in the thermal and kinetic energy of the flame, resulting in increased porosity in the coating. This behavior can be attributed to the low oxide content of the coating, the higher aluminum content, and the richer NiAl phase, which serves as a reservoir for aluminum.

Preparation and interdiffusion behaviour of a NiCrAlY coating with high entropy ceramic diffusion barrier

In order to solve the problem of elemental interdiffusion between the superalloy and coating, the diffusion barrier of (AlCoCrNiRe) and (AlCoCrNiRe)O between the N5 superalloy and NiCrAlY coating was prepared by magnetron sputtering. Isothermal oxidation behaviour of the NiCrAlY coatings with diffusion barrier was evaluated at 1050 °C. The results show that the application of (AlCoCrNiRe) and (AlCoCrNiRe)O diffusion barriers significantly reduced the precipitation of TCP phase in the substrate. (AlCoCrNiRe)O high entropy ceramic diffusion barrier has a better effect of inhibiting interdiffusion because of its own properties and the oxides formed in situ. Furthermore, the NiCrAlY coating with (AlCoCrNiRe)O high entropy ceramic diffusion barrier provides higher resistance to high-temperature oxidation for the substrate.

Effect of Zr/Dy on thermal corrosion resistant properties of NiCrAlY coatings

Dy or Zr doped NiCrAlY coatings were fabricated via arc ion plating technology on nickel-based single crystal superalloy. Thermal corrosion tests of NiCrAlY, NiCrAlYZr, and NiCrAlYDy coatings with 75 wt% Na2SO4 + 25 wt% NaCl mixed salt were conducted at 900 °C. The weight gain of the NiCrAlY, NiCrAlYZr, and NiCrAlYDy coating samples after 100 h of thermal corrosion was as follows: −11.32, 1.06 and −6.56 mg∙cm−2. The results indicated that the NiCrAlYZr coating exhibits superior thermal corrosion resistance compared to both NiCrAlY and Dy-doped NiCrAlY coatings due to the beneficial effects of Zr interacting with S and Cl, making it more effectively protect the hot components of aircraft engine from thermal corrosion.

Oxidation Resistance of NiCrAlY Coating Prepared by Arc Ion Plating

Oxidation Resistance of NiCrAlY Coating Prepared by Arc Ion Plating

High-temperature structural evolution and mechanical properties of oxygen-containing NiCrAlY coatings

NiCrAlY is utilized as a protective coating for gas turbine components at high temperatures. Enhancing the erosion resistance of NiCrAlY coatings can be achieved by optimizing their mechanical properties. In this study, a NiCrAlY coating containing oxygen was deposited by magnetron sputtering and investigated concerning microstructural evolution at 900° C. Oxygen inclusion does not increase the hardness of the NiCrAlY coating in its initial deposited form. When annealing at 900 °C for 1 h, the NiCrAlYO coating develops into a nanocrystalline structure, where Y2O3 is evenly distributed inside the γ/γ′ matrix. This leads to the phenomenon of age-hardening, causing the hardness to increase to 12.4 GPa. Conversely, the hardness of the O-free NiCrAlY coating sharply declines to 9.3 GPa. Moreover, the wear resistance of the NiCrAlYO coating has been enhanced at a temperature of 700 °C, owing to its exceptional mechanical stability.

A study on the thermal cyclic behavior of NiCrAlY coatings via femtosecond laser remelting with high repetition rate

NiCrAlY coatings are widely applied bond coatings in thermal barrier coating systems to prevent high-temperature oxidation of turbine blades. However, the uncontrollable microstructure design and high surface roughness seriously affect the oxidation resistance. Herein, NiCrAlY coatings fabricated through arc ion plating were subjected to high-repetition-rate femtosecond laser remelting (HRR-FLR) treatment. This study assessed the surface rumpling behavior, residual stress evolution, and thermally grown oxide (TGO) layer growth under thermal cyclic oxidation at 1100 °C to determine their impact on coating lifetime. The results indicated that the HRR-FLR treatment was effective in reducing surface roughness, restructuring microstructures, and redistributing reactive elements. These improvements facilitated the formation of a flat, continuous, and dense alumina layer, enhancing the spallation and rumpling resistance of TGO. Consequently, the efficiency of HRR-FLR technique was proven in significantly improving the lifetime of NiCrAlY coatings, which provides a new application prospect of femtosecond lasers in remelting and polishing of metallic surfaces.

Comparison of oxidation behavior of NiCrAlY coatings before and after surface aluminizing modification

Surface aluminizing of the coating can effectively balance its high-temperature oxidation resistance and mechanical properties, while also minimizing the rumpling in the oxide layer caused by the phase transition. In this study, NiCrAlY coatings prepared using arc ion plating were aluminized with different Al content, which was reflected by the thickness of the Al layer deposited via magnetron sputtering. The effects of the aluminizing treatment on high-temperature oxidation resistance was investigated. After 150 h of oxidation, the original coating exhibited complete failure, evident through the flaking of the oxide layer and severe internal oxidation. The aluminizing treatment delayed the formation of an Al-depletion zone within the coating, which effectively prolonged the high-temperature durability of the coatings. Moreover, the different levers of Al content also influenced the thickness of the subsequently generated oxide layer.

Oxidation behavior of NiCrAlY coatings with man-made IDZ fabricated by arc ion plating at 1050 °C

Oxidation behavior of the NiCrAlY coatings with/without man-made interdiffusion (M-IDZ) layers at 1050 °C is investigated. Results indicate that four kinds of the M-IDZs are designed, which are based on the chemical composition of the IDZ, and can be applied between the single crystal superalloy and the NiCrAlY coating. Their additions do not affect the formation and thickening of the α-Al2O3 scales, and show excellent oxidation resistance. After oxidation, the thicknesses of the changed superalloys beneath the coatings with M-IDZs are much less than the case of the NiCrAlY coating without M-IDZs, weakening the side effects of the element interdiffusion on the superalloy. At the same time, the existence of the M-IDZs can accelerate the transformation from needle-like σ and μ phases to P phase.

Oxidation behavior at 1100 °C of NiCrAlY coatings deposited by various PVD techniques

NiCrAlY has been widely used as a protective layer on gas turbine engine hot section components and can be synthesized by different physical vapor deposition techniques. Here, the oxidation behaviors of arc-evaporated, magnetron-sputtered, and hybrid arc/sputtering NiCrAlY coatings were investigated. Preparation techniques can significantly affect the oxidation behavior of NiCrAlY coatings by adjusting the Al content. Arc-evaporated NiCrAlY coating has the lowest resistance against oxidation at 1100 °C, forming a bi-layered scale with outer spinel-rich and inner alumina-rich oxides. In comparison, the other two NiCrAlY coatings with higher Al content were oxidized to form dense scales enriched in α-Al2O3 after 250 h of oxidation. Furthermore, the outward diffusion of Mn from Hastelloy X promotes the production of spinel oxides, which accelerates the oxidation process.

Microstructure evolution and cyclic oxidation performance of Cr2AlC as active diffusion barrier for NiCrAlY coating on TiAl alloy

Cr2AlC diffusion barrier was prepared between NiCrAlY coating and TiAl alloy. Long-term cyclic oxidation was conducted at 900 °C in air for 1000 cycles(h). Cr2AlC and transformed Cr7C3, α-Cr and TiC had significant blocking effects on Ti diffusion. So, the Al-rich phase in the NiCrAlY coatings was NiAl rather than Ni2TiAl, which inhibited the generation of TiO2 and avoided the presence of α-Cr in the Al2O3 scales. As a result, the stability and spalling resistance of the Al2O3 scales were enhanced.

Preparation and oxidation behavior of NiCrAlY coatings with Re-base diffusion barrier on a nickel-based single-crystal superalloy

Four Pt-modified NiCrAlY coatings with different thicknesses and Re contents diffusion barrier (DB) were prepared on a single crystal superalloy by sequential treatments of Ni-Re co-electrodeposition, arc ion plating and Pt electrodeposition. To explore the effect of different forms of the Re-base DB on the oxidation resistance and interdiffusion behavior in Pt-modified NiCrAlY coating, the isothermal oxidation behavior of the four coatings at 1050 °C was investigated. The results indicated that Re-base DB can effectively improve the high-temperature oxidation resistance of Pt-modified NiCrAlY coating by inhibiting the internal diffusion of Al as well as the external diffusion of refractory elements. The coating samples of 5 µm Ni-45Re (at%) DB exhibited the lowest interdiffusion extent and the best long-term stability. Higher Re content DB tended to form layered σ-(Cr,Re) phase, while DB with lower content could only form granular σ-(Cr,Re) phase. The influence of different micromorphologies of the σ-phase on the long-term stability of DB was discussed.

Investigation on fretting wear of NiCrAlY coatings at elevated temperature

Tribological properties of MCrlAlY (M = Ni, Co, or Ni and Co) coatings are well established under sliding conditions. However, the tribological properties of MCrAlY coatings under fretting conditions, typically encountered in blade-disk dovetail connection of aero engines, are yet to be further studied. The present study aims to investigate the elevated-temperature fretting wear behaviors and mechanisms of a NiCrAlY coating deposited on a γ-TiAl substrate. Scanning electron microscope, energy dispersive spectrometer, and Raman spectrometer were utilized to study the wear scars and tribolayers. Transmission electron microscope and nanoindentation tests were utilized to elucidate the structural and mechanical properties of the tribolayers. The results revealed that, large areas of sea-shape tribolayers formed on the NiCrAlY coating and resulted in the low wear rate and friction coefficient. While for the γ-TiAl substrate, small areas of island-shape tribolayers formed, and insufficient to protect the alloy from severe abrasive wear, delamination, and adhesive wear. The formation of the sea-shape tribolayers was found to be derived from the high diffusion coefficients in the oxides. In addition, the wear of the counterpart nickel alloy was not aggravated with the NiCrAlY coating, since sea-shape tribolayers also formed on the counterpart.

Study on Microstructure and Oxidation Resistance Mechanism of Y-Modified NiCrAlY Coating Prepared by Pack Cementation

The existence mode of Y is an important factor affecting the oxidation resistance of NiCrAlY coating. The traditional thermal sprayed NiCrAlY coating had some problems such as internal-stress-induced coating peeling, when it was used at high temperature for a long time. In this paper, Y-modified NiCrAlY coatings were prepared on the surface of thermal-sprayed NiCrAlY coatings by halide-activated pack-cementation technology. The microstructure and oxidation resistance of the coatings after thermal insulation at 1100 °C for 100 h were analyzed. The results show that the oxidation resistance of Y-modified NiCrAlY coatings was improved significantly. When the oxidation temperature and time were 1100 °C and 100 h, the oxidation weight gain of thermal sprayed NiCrAl0.5Y and NiCrAl1.0Y coatings was 3.8 and 2.2 times that of Y-modified coatings, respectively. In addition, the NiCrAlY coatings prepared by pack cementation had better oxidation resistance than thermal sprayed coatings when the content of Y was equal. The oxidation resistance of the pack-cemented NiCrAlY coatings were increased with the increase in Y content when it was in a range of 0~1 wt.%. When oxidized at 1100 °C for 100 h, the oxidation weight gain of the pack-cemented NiCrAl0.5Y and NiCrAl1.0Y coatings was about 1.8 and 0.9 mg/cm2, respectively. The formation of highly stable intermetallic compounds Ni5Y in the pack-cemented Y-modified coatings is concluded to be the reason for improving the oxidation resistance of the coatings. This work can provide a solution to improve the oxidation resistance of traditional MCrAlY coatings.

The Oxidation Properties of a NiCrAlY Coating Fabricated by Arc Ion Plating

The microstructures, phase compositions, and high-temperature oxidation behavior of a NiCrAlY coating fabricated by arc ion plating were investigated by scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), atomic force microscope (AFM), and X-ray diffractometer (XRD). The results indicate that the NiCrAlY coating was covered by a protective Al2O3 scale with excellent oxidation resistance after oxidation at 1050 °C for 100 h; the β phase rich in Al in the as-deposited coating was transformed into a γ/γ’ phase; the interdiffusion zone (IDZ), secondary reaction zone (SRZ), and needle-like TCP phases were detected in the superalloy substrate beneath the coating.

Excellent thermal shock resistance of NiCrAlY coatings on copper substrate via laser cladding

• NiCrAlY layers were prepared on copper substrate via laser cladding technology. • Metallurgical bonding and excellent thermal shock resistance are obtained. • Lifetime of this NiCrAlY layers is much longer than NiCrAlY by other methods. Coatings are widely used to protect substrates in extreme thermal environments (e.g., arc heaters), and thermal shock resistance is a crucial parameter for the coatings, which requires tight interlayer bonding between coatings and substrates. In this work, NiCrAlY coatings were highly required for the pure copper substrate to restrict the electric arc in arc heaters. To overcome the bonding difficulty of coating on the copper surface, the NiCrAlY coatings were prepared by two laser cladding methods: conventional laser cladding (CLC) and high-speed laser cladding (HSLC). The microstructure, composition, and thermal shock resistance of NiCrAlY cladding layers prepared by both methods were investigated. Benefitting from the high cooling rate and high energy density, the HSLC-layer has better composition uniformity and tighter interlayer bonding than the CLC-layer, achieving a 30%–45% improvement in thermal cycling lifetime. Besides, the NiCrAlY layers prepared on copper substrate by both laser cladding methods exhibit 3–10 times better thermal shock resistance than those NiCrAlY layers prepared by conventional spraying methods. It further confirms the great effects of metallurgical bonding and composition uniformity on the thermal shock resistance of coatings. The NiCrAlY layers fail in the form of internal cracking and interface peeling, and the corresponding failure mechanism is discussed.

Synergistic effect of Re-base diffusion barrier and Pt modification on the oxidation behaviour of NiCrAlY coatings

The cyclic oxidation behaviour at 1050 °C and isothermal oxidation behaviour at 1100 °C of a low-diffusion Pt-modified NiCrAlY coating with Re-base diffusion barrier were investigated, compared with conventional NiCrAlY, Pt-free NiCrAlY with Re-base diffusion barrier and Pt-modified NiCrAlY without Re-base diffusion barrier coatings. The results revealed that Pt-modified NiCrAlY coating with Re-base diffusion barrier exhibited the best oxidation resistance and lowest interdiffusion extent. The role of Re-base diffusion barrier in inhibiting elemental interdiffusion and the contribution of Pt to the growth rate and adhesion of Al2O3 scale, as well as the long-term stability of Re-base diffusion barrier, are discussed.

Interface stability of NiCrAlY coating without and with a Cr or Mo diffusion barrier on Ti-42Al-5Mn alloy

NiCrAlY coatings without and with a Cr or Mo diffusion barrier were fabricated on Ti-42Al-5Mn alloys by vacuum plasma spray. The microstructure evolution and inter-diffusion behavior were studied via isothermal oxidation tests conducted at 950 °C. The results revealed that substantial inter-diffusion happened in the coating without a diffusion barrier, accelerating the degradation of NiCrAlY coating. While the addition of Cr or Mo diffusion barrier deferred the inter-diffusion phenomenon, and Mo performed much better than Cr due to its high stability. The thickness of the inter-diffusion zone with the Mo diffusion barrier was just about one-fifth of that for NiCrAlY coating after 200 h oxidation. The results could provide comprehensive understanding of the microstructural evolution of NiCrAlY coating, which was essential for controlling the inter-diffusion behavior and improving the durability of NiCrAlY coating.

Thermal shock resistance of a NiCrAlY-coated Alloy 625 system produced by laser powder bed fusion

Additive Manufacturing offers an innovative route for producing high-quality parts in various fields. A bi-material system, consisting in a NiCrAlY bond coat deposited onto a Ni-based Alloy 625 substrate, was manufactured by laser powder bed fusion (LPBF). Test samples were prepared and included sol-gel ceramic ZrO2(Y2O3) top coats as well as a specific grain boundary serration (GBS) heat treatment for promoting high temperature resistance. These specimens were subjected to very severe thermal shock cycles between 950 °C and 300 °C, characterized by steep heating and air quenching rates in a state-of-the-art burner rig designed to render gas turbine conditions, and their integrity was compared. While LPBFed NiCrAlY coatings were relatively spared from degradations due to thermal shocks, ceramic top coats exhibited clear spallation. Poor bonding was particularly experienced by specimens subjected to the GBS heat treatment due to the unavoidable formation therein of surface oxides. Numerous cracks were detected within NiCrAlY bond coats, both in tested specimens as well as in pre-cycled as-built ones, which suggested a dominant role of the LPBF process known for generating residual stress. Heat treated specimens exhibited nearly no cracking. Hardness was found to significantly increase within as-built NiCrAlY bond coats as a result of heat exposure during thermal shock cycling and was attributed to precipitations. The fully recrystallized microstructure of heat treated specimens, on the other hand, was found more stable. The present study completes a series of investigations demonstrating the great potential for manufacturing excellent high temperature structural components by means of LPBF as opposed to more constraining conventional routes.

CHARACTERIZATION OF ELECTRODEPOSITED ZIRCONIA MODIFIED NiCoCrAlY COMPOSITE COATINGS ISOTHERMALLY OXIDIZED AT 1000∘C

The objective of this study is to evaluate the electrodeposited zirconia (ZrO2)-modified NiCoCrAlY composite coatings subjected to isothermal oxidation at 1000∘C under atmospheric conditions for 24 h. NiCoCrAlY composite coatings with 0, 10, 40, and 60[Formula: see text]wt.% of cobalt contents have been compared. The oxidation studies have revealed that the doping of yttrium (Y) and ZrO2 results in a significant reduction in weight gain, thereby imparting improved oxidation resistance. The morphology of the oxides formed on the surface is analyzed using field emission scanning electron microscope and the phases are identified by X-ray diffraction studies. The major oxidation products formed on the surface of NiCrAlY and Ni10CoCrAlY coatings consist of NiO. In contrast, the surface of Ni40CoCrAlY coating comprises of predominantly CoO and small amount of NiCo2O4. Cobalt oxide (CoO) is formed as the major phase on the surface of the Ni60CoCrAlY coating. The grains on the surface of ZrO2-modified NiCoCrAlY composites are faceted, while such grains are not seen in the case of unmodified coatings. The texture analysis reveals that cooling rate is the deciding factor for the facet formation. The coatings have been characterized extensively using X-ray photoelectron spectroscopy to get in-depth understanding.

Studying the Erosion Corrosion Behavior of NiCrAlY Coating Layer Applied on AISI 446 Stainless Steel Using Thermal Spray Technique

Erosion-Corrosion behavior of NiCrAlY coatings deposited on 446 stainless steel samples by flame thermal spraying was performed in this research. The hardness of uncoated and coated sample is measured, using HVS-1000 Digital Micro hardness Tester. The results of hardness show that the coated samples have higher hardness than uncoated samples. Microstructure and phase analysis of the coating layer was observed by using optical microscope, X-ray diffraction (XRD), and SEM respectively. Erosion –Corrosion test was performed in different period of time (6, 12, and 18 hr.) using the erosion-corrosion system. The result shows that because of uniform distribution and homogeneous of coating layer, weight loss was very low and approximately equal in all period of testing time.